1. Technical Field of the Invention
This invention relates to optics primarily used with lasers. More particularly, it relates to a process for making retardation plates such as quarter wave plates for use with molecular lasers such as CO2. This process utilizes deformed zinc selenide.
2. Description of the Prior Art
Industrial lasers such as CO2 lasers quite often require the use of circularly polarized light for cutting and welding of metals. There are various ways known to those skilled in the art to force the industrial lasers to lase linearly polarized. This linearly polarized light is then converted to circularly polarized light utilizing an external optical device. The most common device for doing this is referred to as a phase retardation reflector. This is a thin film total reflector which converts linearly polarized light to circularly polarized light when the proper reflecting angle conditions are met. The incident angle requirements usually results in the reflected laser beam propagating in an undesirable direction thereby requiring an additional reflector to straighten out the optical path. These two reflectors (and the associated mirror mounts) can be expensive and are a possible source of point instability problems.
Another way of converting linearly polarized light to circularly polarized light is to use a transmissive quarter wave plate. Before this current invention, the most common material used at 10.6 micron wavelength was cadmium sulfide. Cadmium sulfide is a crystalline material which exhibits a birefringence. Cadmium sulfide exhibits several problems which include high material cost, unavailability of large sizes, and high absorption compared to zinc selenide. Another undesirable feature is associated with the fact that cadmium sulfide exhibits a large birefringence which would require a theoretical thickness of 0.25 mm to achieve a quarter wavelength retardation of a CO2 laser beam. Because this is too thin to be practical, these retardation plates are made in an odd number of quarter wavelength multiples. Most commonly a retardation of 5 quarter wave or 7 quarter wave is used. This, in turn, creates other problems such as alignment, temperature and wavelength sensitivity.
By far, the most common transparent material used with high power CO2 lasers is zinc selenide. This is a very uniform material typically grown from chemical vapor deposition. As grown, it shows no obvious birefringence. It has been observed that there is a certain type of damage which can occur in zinc selenide where the transmissive optical element (output coupler or lens) becomes absorbing and stressed. The damaged optical component sometimes also exhibits a small radial birefringence (visible light) which is not uniform. This is considered an undesirable effect. Finally, in the prior art, it is known that some clear plastic materials can be made to be sufficiently birefringent to form quarter wave plates for visible light if the plastic is elongated in one direction. However, no organic material can transmit a powerful CO2 laser beam.
It would be very desirable if a quarter wave plate for use at 10.6 micron wavelength could be made out of zinc selenide. While zinc selenide has exhibited a small amount of birefringence which is apparent at visible wavelengths, this effect is uncontrolled and associated with damaged optics.